Heretofore, a polymer having fluorosulfonyl groups (—SO2F) in a copolymer of a fluorinated monomer of the following formula with tetrafluoroethylene, converted to sulfonic acid groups (—SO3H), has been commonly used as an electrolyte material constituting an ion change membrane (such as a membrane to be used for electrolysis of sodium chloride or for a polymer electrolyte fuel cell) or a catalyst layer for a fuel cell. In the formula, Y represents a fluorine atom or a trifluoromethyl group, n is an integer of from 1 to 12, m is an integer of from 0 to 3, and p is 0 or 1, provided that m+p>0.CF2═CF—(OCF2CFY)m—Op—(CF2)n—SO2F
A sulfonic acid group-containing polymer (hereinafter referred to also as a sulfonic acid polymer) is a polymer which is capable of reducing the electric power for electrolysis when used in e.g. an electrolytic cell for sodium chloride in the form of a membrane having a high ion exchange capacity. Further, in a case where such a sulfonic acid polymer is used for a fuel cell, it is a polymer capable of improving the power generation energy efficiency. As such a sulfonic acid polymer, preferred is a polymer having a larger ion exchange capacity and a lower electric resistance.
A conventional fluorosulfonyl group-containing monomer is required to be copolymerized with tetrafluoroethylene having a high polymerization reactivity in order to obtain a perfluoropolymer having a high molecular weight. However, if it is attempted to increase the ratio of the fluorosulfonyl group-containing monomer to be used for copolymerization for the purpose of increasing the ion exchange capacity of the sulfonic acid polymer, there has been a problem that the molecular weight of the copolymer tends to be low. A membrane formed of a copolymer having a low molecular weight has had a problem that the mechanical strength and durability are inadequate, and as such, is not practically useful.
Under the circumstances, in order to maintain a high ion exchange capacity and to obtain a membrane having a high tetrafluoroethylene content to secure the strength, it has been proposed to use a monomer having two sulfonic acid groups or precursor groups capable of being converted to sulfonic acid groups, such as fluorosulfonyl groups, in its molecule. Namely, the following monomer is proposed in Patent Document 1:[FSO2(CF2)a][FSO2(CF2)b]CF-QF-CF2OCF═CF2 
In the above formula, a is an integer of from 1 to 3, b is an integer of from 1 to 3, and QF is a single bond or a C1-6 perfluoroalkylene group which may have an etheric oxygen atom.
To produce such a monomer, it is necessary to substitute fluorine atoms for all hydrogen atoms in an intermediate containing a structure of [FSO2(CH2)a][FSO2(CH2)b]CH— by using fluorine gas, but such a process has had a problem that during the fluorination reaction, C—S bonds are likely to be broken, and the yield of fluorination is low. Further, there is no disclosure about a polymer obtained by polymerizing this monomer.
The following monomer is proposed in Patent Document 2.(XSO2)kCY1(CF2)mO(CFZCF2O)nCF═CF2 
In the above formula, k=2 or 3, k+1=3, m=0 to 5, n=0 to 5, X=F, Cl, OH, O(M)1/L (wherein M is a monovalent to trivalent metal, and L is the valence of the metal), OR (wherein R is a C1-5 alkyl group, and such an alkyl group may be one containing an element other than carbon or hydrogen), or A-(SO2Rf)aB (wherein A is nitrogen or carbon, as to a, a=1 when A is nitrogen and a=2 when A is carbon, B is hydrogen or a monovalent metal, and Rf is a perfluorinated alkyl group), Y═F, Cl or CF3, and Z=F, Cl, CF3, Br or I.
A sulfonic acid polymer obtainable from such a monomer has a structure wherein a plurality of sulfonic acid groups are bonded to one carbon atom, and there is concern about stability, as there is no information as to whether or not such a structure may have durability over a long period of time.
Patent Document 1: WO2005/003062 (claim 17)
Patent Document 2: WO03/106515 (claim 1)